Process of and apparatus for casting metals without sprues

ABSTRACT

Liquid metal is supplied from the bottom to the top into a casting mold by way of a casting runner and a filling hole arranged at the lowest point of a casting mold. Direct further use of the casting by avoiding formation of a sprue that must be subsequently processed is possible due to the fact that after totally filling the casting mold and only partial solidification of the metal therein, the casting pressure is decreased or turned off, so that liquid metal flows back out of the casting mold only through a return flow channel having a cross section significantly smaller than the cross section of the casting runner. A ram having a face corresponding in size and shape to the outer contour of the casting in the region of the filling hole is pushed at least over the last segment of the casting runner, thus displacing liquid metal present in the last segment into the casting mold sufficiently far in the direction of the mold interior until the fact of the ram aligns with the inner contour of the casting mold.

BACKGROUND OF THE INVENTION

The invention relates to a process for casting metals, in particularalloys with low melting points, during which process liquid metal isfilled from the bottom to the top into a casting mold by way of acasting runner and a filling hole arranged at the lowest point of thecasting mold, and to an apparatus to effect such a process.

Such processes are employed, for example, during the production of lostcores for injection molded plastic parts. During such process topcasting must be avoided, so that the casting takes place by necessity atthe lowest point of the mold. To date the liquid metal during theprocess of the aforementioned kind has been filled under pressure intothe casting mold and the casting pressure has been maintained until themetal has totally solidified in the casting mold. After the metal hassolidified, the resulting sprue or runner has to be removed and thefilling point has to be ground smooth.

The cores produced in this manner are used, among other things, for massproducing plastic parts in the automobile industry, where it isimportant on account of high quantity requirements to hold the cost ofproduction as low as possible. However, these costs are significantlyincreased by the requisite subsequent treatment and the additionallyrequired machines or manual activity.

SUMMARY OF THE INVENTION

Therefore, the object of the invention is to develop a process andapparatus by which a casting can be produced without sprues or runnersand can be directly further used without subsequent processing.

This object is achieved according to the invention by decreasing orturning off the casting pressure after totally filling the casting moldand only partial solidification of the metal in the casting mold, sothat the liquid metal flows back out of the casting mold by way of areturn flow channel having a cross section significantly smaller thanthe cross section of the casting runner. A ram having a facecorresponding in size and shape to the outer contour of the casting inthe region of the filling hole is pushed at least over the last segmentof the casting runner, thus displacing the liquid metal present thereininto the casting mold as far in the direction of the mold interior untilthe fact of the ram aligns with the inner contour of the casting mold.Due to the adaptation of the fact of the ram to the desired outercontour of the core in the region of the filling hole, the core islocated now in a totally closed casting mold and solidifies withoutforming a casting sprue that must be subsequently processed.

When advancing the ram and closing the filling hole at the end of thecasting operation, the ram displaces the liquid material, a process thatwould not have been possible with the conventional process since themold with the liquid metal is filled totally without air pockets.However, a cavity, into which the ram can be subsequently pushed uponclosing the filling hole of the liquid metal, is created according tothe invention in the casting mold by means of the return flow of theliquid metal out of the casting mold after reducing or switching off thecasting pressure. The return flow volume is controlled by the crosssection of the return flow channel being significantly smaller than thecross section of the casting runner. In this manner it is ensured thatuntil the filling hole is closed by the ram, not too much liquid metalhas flowed back and the cavity within the casting mold is not too large.

Upon pressing the metal into the casting mold, the ram closes accordingto the invention the casting runner before the face of the ram alignswith the inner contour of the casting mold. In this manner it isachieved that the metal displaced by the ram is forced reliably into thecasting mold and is not discharged partially by way of the castingrunner, thus preventing a defined filling of the cavity in the castingmold.

To achieve maximum filling of the cavity produced in the casting mold bymeans of the return flow of the metal, it is provided in anotherembodiment of the inventive idea that the volume of the metal that hasflowed back from the casting mold is greater than or equal to the volumeV_(s) of the metal displaced by the ram into the interior of the castingmold. A residual cavity, which always remains eventually in the regionof the still liquid metal in the interior of the casting mold, itmaintained very small by means of the process according to the inventionand does not limit the use of the casting in any way.

Defined conditions for the liquid metal flowing back and the closing ofthe filling hole are achieved expediently by moving the ram by means ofrandom time control. In so doing, the period of time to fill the castingmold by way of the casting runner with metal amounts to approximately 20seconds, the period of time from the end of filling the casting moldwith metal until reducing or switching off the casting pressure amountsto approximately 40 seconds, and the entire period from the start of thecasting operation until the closing of the filling hole by means of theram amounts to approximately 80 seconds. It has turned out that uponobserving these recommended times, the cavity remaining in the castingmold can be held very small or in the ideal case can be even totallyavoided.

The invention also relates to an apparatus to carry out the aboveprocess. Such device includes a nonreturn valve, opening in thedirection of casting, disposed in the casting runner outside the segmentover which the ram passes. A return flow channel, acting as a bypasschannel for the nonreturn valve in the casting runner and bridging thenonreturn valve, has a cross section that is significantly smaller thanthe cross section of the casting runner. By means of the nonreturnvalve, the liquid metal is prevented from flowing back through thecasting runner after the casting pressure has been switched off, so thatonly a relatively small quantity of liquid metal can flow back throughthe return flow channel bypassing the non-return valve. To achieve aslow, defined return flow of the melt, it has proven to be expedientthat the ratio of the cross section of the return flow channel to thecross section of the casting runner ranges in an order of magnitude of1:10 to 1:50. Preferably this cross section of the return flow channelcan be modified, e.g., by means of a setscrew, in order to be adaptedoptionally to different conditions when casting different alloys.

The solution according to the invention is characterized by the factthat the casting runner includes a segment which opens into the fillinghole and, when seen in the direction of filling, is last, and whichextends coaxially to the ram and its direction of displacement. Further,the last segment of the casting runner has an entry segment, which isinclined laterally and in which is disposed the nonreturn valve.

The ram is guided preferably in a bore of the casting mold which extendscoaxially to the last segment of the casting runner, so that the ram isconveyed exactly to the filling hole of the casting mold. Since the faceof the ram is adapted to the desired outer contour of the casting, whichmay or may not be rotationally symmetrical, the ram preferably is guidedin the bore to not be twistable or rotatable therein. The forcesrequired to displace the liquid metal are generated according to theinvention by the fact that the ram can be driven hydraulically.

In a preferred embodiment it is ensured that the casting runner isclosed after the casting pressure has been switched off by the fact thatthe nonreturn valve is pressed by a pull back spring against a valveseat.

The metal has to be kept liquid in the casting runner in order not toobstruct the ram while closing the filling hole. Therefore, thetemperature in the casting runner has to be greater than or equal to themelting temperature of the alloy to be cast, while the temperature ofthe casting mold, in which the casting is supposed to solidify, has tobe less. Therefore, in another embodiment of the invention it isprovided that a significant proportion of the casting runner and the ramare arranged in a housing which is connected detachably to the castingmold with a thermal barrier therebetween. In this manner, the loss oftemperature from the housing to the casting mold is reduced.

BRIEF DESCRIPTION OF THE DRAWING

Improvements, advantages and possible applications of the invention willbe apparent from the following description of one embodiment and theaccompanying drawing. At the same time, all of the described and/orillustrated features form by themselves or in any arbitrary logicalcombination the subject matter of the invention, independently of theirsummary in the claims or their references.

The sole FIGURE is a diagrammatic drawing of a casting apparatusaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Liquid metal, in particular an alloy with a low melting point, is filledfrom the bottom to the top into a cavity within a casting mold 3,comprising two mold halves 1, 2, by way of a casting runner 4 and afilling hole 5, arranged at the lowest point of the casting mold 3. Theliquid metal is forced under pressure into the casting mold 3, duringwhich process the air contained in the casting mold 3 escapes through agap 6 existing between the mold halves 1, 2. The gap 6 is too small forthe liquid metal also to flow therethrough. In this manner it is ensuredthat no unwanted deposit forms on the joint face of the casting mold 3.

After totally filling the casting mold 3, the metal contained in thecasting mold 3 begins to solidify to form a casting 7, and in particularstarting at the outer edges of the casting 7 and progressing toward theinterior. In so doing, lines 8, 9 in the drawing denote lines ofidentical solidification. After a predetermined period of time, inparticular when only a relatively small volume of metal in the interiorof the mold is still liquid, the casting pressure is switched off. Uponswitching off the casting pressure, the liquid metal flows, subject tothe action of gravity, out of the casting mold 3. However, the liquidmetal is prevented from flowing back through the casting channel due tothe fact that the casting runner 4 has a nonreturn valve 10, which opensin the direction of casting and having a valve body 24 that is forcedagainst a valve seat 12 by a pull back spring 11 and thus closes thecasting runner 4 upon lack of casting pressure. Therefore, the liquidmetal can flow back only through a return flow channel 13 which bypassesthe nonreturn valve 10 and acts as a bypass channel and whose crosssection is significantly less than the cross section of the castingrunner 4. Preferably the ratio between the cross section of the returnflow channel 13 and the cross section of the casting runner 4 is on theorder of magnitude of 1:10 to 1:50. The cross section of the return flowchannel 13 can be modified, e.g., by way of a setscrew 14 in order to beable to adapt, if desired, to different alloys and casting volumes.

The casting runner 4 is subdivided into a last segment 16 opening intothe filling hole 5 and an entry segment 17, which is inclined laterallyand in which the nonreturn valve 10 is arranged. A ram 18, which isguided in a bore 19 of the casting mold 13, can be displaced coaxiallyto the last segment 16 of the casting runner 4, bore 19 extendingcoaxially to the last segment 16 of the casting runner 4. A face 20 ofthe ram 18 corresponds in size and shape to the contour of the of thecasting 7 in the region of the filling hole 5. Since this contour of thecasting 7 is usually not rotationally symmetrical, the ram 18 is guidedin the bore 19 to not be twistable or rotatable therein.

Upon switching off the casting pressure, one portion of the liquid metalflows back out of the casting mold 3 into the casting runner 4, thusproducing a cavity 15 in the casting 7. The metal flows back very slowlythrough the small cross section of the return flow channel 13, so thatthe size of the resulting cavity 15 can be accurately defined throughtime control. The liquid metal present in the last segment 16 of thecasting runner 4 is forced back into the casting mold 3 by means of theram 18 when ram 18 is advanced. Through time control the volume of metalthat has flowed back and thus the volume of the resulting cavity 15 canbe tuned accurately to that of the metal forced back into the castingmold 3 by means of the ram 18, so that the cavity 15 is totally refilledor at least almost totally refilled.

The fact 20 of the ram 18 corresponds in size and shape to the outercontour of the casting 7 in the region of the filling hole 5. Thus, theinner contour of the totally closed casting mold 3 upon closing thefilling hole 5 by means of the ram 18 corresponds exactly to the desiredouter contour of the casting 7. Therefore, no casting runner or sprue isformed, so that the casting 7 upon removal from the casting mold 3 canbe further used without subsequent processing.

The ram 18 is driven by means of a hydraulic drive 21. A significantportion of the casting runner 4 and the ram 18 is arranged in a housing22, which is connected detachably to the casting mold 3 with a thermalbarrier 23 therebetween.

We claim:
 1. A process for casting molten metal to form a metal casting,said process comprising:filling molten metal under a casting pressureinto a cavity of a casting mold through a casting runner and a fillinghole located in a lowest position of said cavity of said casting mold;after said cavity is totally filled with molten metal and after onlypartial solidification of said metal in said cavity, decreasing orstopping said casting pressure, whereby molten metal not yet solidifiedwithin said cavity flows outwardly therefrom through a return flowchannel having a cross section significantly smaller than the crosssection of said casting runner; and displacing molten metal thuswithdrawn from said cavity and present in a last segment of said castingrunner adjacent said cavity by pushing said withdrawn molten metal fromsaid last segment back into said cavity with a ram having a facecorresponding in size and shape to an outer contour of said metalcasting to be formed in the region of said filling hole until said faceof said ram aligns with an inner contour of said cavity of said castingmold.
 2. A process as claimed in claim 1, wherein said pushing causessaid ram to close said casting runner before said face of said ramaligns with said inner contour of said cavity of said casting mold.
 3. Aprocess as claimed in claim 1, wherein said molten metal flowed fromsaid cavity has a volume greater than or equal to a volume of saidmolten metal displaced back into said cavity by being pushed by saidram.
 4. A process as claimed in claim 1, comprising controlling movementof said ram by a selectable time control.
 5. A process as claimed inclaim 1, comprising conducting said filling such that a period of timeto fill said cavity is approximately 20 seconds.
 6. A process as claimedin claim 1, comprising conducting said decreasing or stopping of saidcasting pressure approximately 40 seconds after completion of saidfilling.
 7. A process as claimed in claim 1, wherein said entireprocess, from the start of said filling until closing of said fillinghole by said ram, comprises approximately 80 seconds.
 8. A process asclaimed in claim 1, comprising regulating said cross section of saidreturn flow channel.
 9. An apparatus for casting molten metal to form ametal casting, said apparatus comprising:a casting mold having a cavityof a configuration of a metal casting to be formed; a casting runnerleading to a filling hole opening into said cavity at a lowest positionthereof, thereby to enable molten metal to be filled into said cavityunder a casting pressure; a return flow channel bypassing at least aportion of said casting runner and having a cross section significantlysmaller than the cross section of said casting runner, whereby, aftersaid cavity is totally filled with molten metal and after only partialsolidification of the metal in said cavity, the casting pressure may bedecreased or stopped, whereupon molten metal not yet solidified withinsaid cavity flows outwardly therefrom through said return flow channel;a ram having a face corresponding in size and shape to an outer contourof the metal casting to be formed in the region of said filling hole;and means for moving said ram from a position withdrawn from said cavitytoward said cavity, such that said ram displaces molten metal that hadbeen withdrawn from said cavity and that is present in a last segment ofsaid casting runner adjacent said cavity back into said cavity, untilsaid face of said ram aligns with an inner contour of said cavity ofsaid casting mold.
 10. An apparatus as claimed in claim 9, furthercomprising a nonreturn valve positioned in said casting runner andoperable to allow molten metal to be supplied under the casting pressurethrough said casting runner in a direction toward said cavity and toprevent flow of molten in an opposite direction through said castingrunner, said return flow channel bypassing said nonreturn valve.
 11. Anapparatus as claimed in claim 10, wherein said nonreturn valve comprisesa valve seat, a valve body movable toward and away from said valve seat,and means to urge said valve body toward said valve seat.
 12. Anapparatus as claimed in claim 9, wherein a ratio of said cross sectionof said return flow channel to said cross section of said casting runneris from 1:10 to 1:50.
 13. An apparatus as claimed in claim 9, furthercomprising means for adjusting said cross section of said return flowchannel.
 14. An apparatus as claimed in claim 13, wherein said adjustingmeans comprises a setscrew.
 15. An apparatus as claimed in claim 9,wherein said last segment of said casting runner extends coaxially ofsaid ram.
 16. An apparatus as claimed in claim 15, wherein said castingrunner further includes an entry segment leading to said last segmentand extending in a direction inclined relative thereto.
 17. An apparatusas claimed in claim 16, further comprising a nonreturn valve positionedin said entry segment.
 18. An apparatus as claimed in claim 15, whereinsaid ram is guided for movement in a bore in said casting mold, saidbore extending coaxially of said last segment.
 19. An apparatus asclaimed in claim 18, wherein said ram is positioned non-rotatably insaid bore.
 20. An apparatus as claimed in claim 9, wherein said movingmeans comprises a hydraulic drive.
 21. An apparatus as claimed in claim9, wherein a significant portion of said casting runner and said ram arewithin a housing detachably mounted on said casting mold with a thermalbarrier positioned therebetween.